1 The meaning conveyed by a declarative sentence, a declarative sentence being one that asserts or denies something, or in other words one that is capable of being true or false. The proposition is not the sentence itself but its meaning. The sentences Snow is white and Schnee ist weiß are obviously different, but they express the same proposition in two different languages, and the proposition is true if and only if snow is indeed white. See also categorical syllogism, mental model, modal logic, possible world, propositional calculus, sentence functor, syllogism, truth functor, truth table, Venn diagram.
2 More generally, any deal proposed for consideration.
3 An invitation to engage in sexual intercourse. [From Latin propositio a setting forth, from proponere to display, from pro before + ponere to place]